<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>字体粗细</title>
  <style>
    h3 {
      font-weight: 400;
    }

    div {
      font-weight: 700;
    }
  </style>
</head>

<body>
  <h3>h3 标题</h3>
  <div>div 标签</div>
</body>

</html>